The present invention relates to a method and a device for transmitting seismic data with a very low error rate.
Modern seismic prospecting methods require transmitting considerable volumes of data. The methods are implemented by arranging in the explored zone numerous data acquisition apparatuses distributed at regular intervals over distances which often amount to several kilometers. Each acquisition apparatus is adapted for collecting seismic signals picked up by one or several appropriate receivers (hydrophones or geophones) in response to vibrations transmitted in the subsoil to be explored by a seismic source and which are reflected by the subsoil discontinuities. The signals the acquisition apparatus collect are sampled, digitized and stored in a memory before being transmitted in real time or in batch mode to a central control and recording station. Transmitting the data between the different acquisition apparatuses and the central station is performed through hertzian channels. Various radio or cable transmission systems are described for example in U.S. Pat. Nos. 4,583,206; 4,979,152; 4,815,044 and in the French patent application FR No. 2,652,162.
For transmitting through hertzian channels, a carrier signal whose phase is modulated in relation to the signal to be transmitted according to the particular mode of transmission of a well-known-type is used: FSK, MSK, PDSK, etc.
The modulating signals are bit streams with sudden variations of amplitude at the successive times of transition between the bits. The main effect of these discontinuities is a notable broadening of the pass-band necessary for each transmission. This is a major drawback, notably because of the restrictions which are often imposed by the regulations applying to telecommunications.
A modulation method, notably known through the article: "Tamed Frequency Modulation, A novel method to achieve spectrum economy in digital transmission" in IEEE Transaction vol. com-26N.degree. 5, May 1978, aims at softening the previous phase transitions and thereby at reducing the pass-band necessary to the transmission of digitized data. This method mainly consists in applying to the carrier wave a phase variation which depends not only on the bit to be transmitted but also on the previous bit and the following bit, with an appropriate weighting. The phase of the carrier therefore undergoes lesser variations, which results in a narrower emission spectrum. Instead of transmitting successive bits, symbols combining three successive bits are transmitted. The achieved combination is a shifting combination since each one of the successive bits to be transmitted is combined with the two adjoining bits.
In spite of the advantage of a narrower pass-band such as provided by this type of modulation known as TFM, it is well-known that it also leads to a certain modification in the distribution of the errors which are then grouped in bursts.
The errors which arise in the transmissions may have very different causes : fading, switching interferences, interchannel crosstalk in the multipath transmission systems, etc. Because of the bit coordinations resulting within the context of a modulation of the TFM type, any possible transmission incident randomly affects one or several ones of the transmitted symbols, and the possible errors are transferred at the time of the decoding to all the bits constituting each one of them.
Numerous methods for coding the data to be transmitted and making it possible to detect and correct possible errors at the reception thereof are well-known. These methods comprise using a block coding process and changing any bit sequence to be transmitted into successive words comprising a group of check bits depending on the selected code: Hamming codes, cyclic codes, codes known as BCH codes, Reed-Salomon codes, Golay codes, etc. Other methods utilize codings of the type known as convolutional where the encoding of the data bits in each block depends on a certain number of previous blocks.
Notably within the framework of the transmission of seismic data, it is particularly useful to detect and correct errors properly. U.S. Pat. No. 4,712,199 for example describes a transmission method in which the series of words to be transmitted is divided into blocks of words containing the same number of signal samples, each one being fitted with synchronizing and identifying signals. By checking these accompanying signals and the number of words in the different blocks, it is easier to locate the instants at which transmission errors occurred and to restore seismic sections in which the seismic events are set in the proper place thereof.
Although this type of checking makes it possible to detect certain errors, the fact remains that it is necessary to reduce the rate of transmission errors to extremely low values and also to reduce the effect of any interference occurring at random on the form of the signal reconstituted after reception or, in some cases, on the form of the signal after certain conventional processing operations. It is for example known that any correlation between a received seismic signal and a transmitted signal, an operation which usually occurs when a vibrator is used as a seismic source, has the effect of amplifying an isolated error on a received signal sample by transferring it to the total correlated trace.